A solid electrolytic capacitor has a fundamental structure constructed by forming a high-density and uniform oxide dielectric film on the surface of a valve-acting metal previously etched to roughen the surface, such as aluminum, tantalum or titanium, forming, for example, an electrically conducting polymer to work out to be a solid electrolyte on the oxide dielectric film, connecting an anode lead wire to the anode terminal (metal surface area having no solid electrolyte) of the valve-acting metal, and connecting a cathode lead wire to the solid electrolyte through an electrically conducting paste. This structure as a whole is then molded with an insulating resin such as epoxy resin to manufacture a solid electrolytic capacitor.
Among the valve-acting metals described above, aluminum is characterized in that the surface area can be easily enlarged by etching and the oxide film formed on its surface by anodization (electrochemical forming) using the aluminum as the anode can be utilized as a dielectric material, as a result, a small-size and large-capacitance solid electrolytic capacitor can be inexpensively produced as compared with other capacitors. Therefore, the aluminum solid electrolytic capacitor is being widely used.
The etching of aluminum is generally performed by the electrolytic etching in an electrolytic solution containing chlorine ion or the like. By this etching, a large number of pores are formed on the surface and the surface area is enlarged. The radius of the pore formed varies depending on the current applied and the etching time but is approximately from 0.05 to 1 μm.
The surface including the pores is then subjected to anodization (electrochemical forming). By this electrochemical forming, a high-density and uniform anode oxide film (dielectric film) having a thickness of approximately from 0.005 to 0.1 μm is formed.
The resulting electrochemically formed aluminum substrate is cut into a predetermined size of a solid electrolytic capacitor. At this time, a protruded portion (bur) remains at the cut end part, however, this exposed aluminum (ground metal) portion is again electrochemically formed as it is to form an anode oxide film (dielectric film) on the cut end part.
On the oxide dielectric film, an electrically conducting polymer is formed to work out to be a solid electrolyte. Then, an anode lead wire is connected or bonded to the anode terminal (metal surface area having no solid electrolyte) of the valve-acting metal using, for example, resistance welding, laser radiation, contact pressure of a caulking or the like, or an electrically conducting adhesive.
However, the bonding using an electrically conducting adhesive takes time in coating the viscous adhesive. Particularly, in the case of stacking and bonding a plurality of capacitor elements, the application is cumbersome. The mechanical bonding using a caulking is not suitable for the case where the bonding portion is small, and the bonding is unstable. The bonding by laser welding has a problem in that the equipment therefor costs highly.
The resistance welding is a method of performing the bonding by fusing a metal in the welding portion utilizing the heat generation (resistance heating) of a dielectric film due to electric resistance. In the case of a material having high electrical conductivity such as aluminum, this resistance is small and heat is less generated. Moreover, because of good thermal conductivity, the bonding portion cannot be satisfactorily fused. Furthermore, in the case of an aluminum foil for low voltage less than 20 V, the dielectric film obtained is thin and has a low resistance value as compared with an aluminum foil for high voltage foil, therefore, the resistance heating hardly occurs. Accordingly, it is difficult to find out good conditions for welding and a problem of connection failure with the anode lead is present.